Added IAM Profile for EC2 driver, and a warning on AWS Profile

IAM roles is a nice feature for restricting instances permissions.
http://docs.aws.amazon.com/IAM/latest/APIReference/Welcome.html

I added a new key on EC2 provider:

    EC2.iam_profile: "iam profile name or iam profile arn"

Now, about AWS driver.

While AWS is based on `libcloud`, and it does implement this, currently I raise an Exception, inviting people to try EC2 driver.

I've also pulled a request to `libcloud` https://github.com/apache/libcloud/pull/150 . When it will be accepted, we will remove the exception and finish the implementation.